Directional ultrasonic transducer with reduced secondary lobes

ABSTRACT

The invention describes a novel design of an acoustic transformer which serves as an impedance matching device between the transducer material and the atmosphere. The transformer serves also to effectively increase the limited maximum permissible amplitude of vibration of the transducer material whereby the maximum acoustic power radiated into the atmosphere is substantially increased. The novel acoustic transformer design also results in a significant reduction in the magnitude of the secondary lobes in the directional pattern thereby additionally improving the performance characteristics of the transducer.

Umted States Patent 1191 1111 3,928,777

Massa Dec. 23, 1975 DIRECTIONAL ULTRASONIC 2,430,013 11/1947 Hansell340/8 MM TRANSDUCER WITH REDUCED 3,513,439 5/1970 Egli 310/82 XSECONDARY LOBES 3,710,151 l/l973 Massa et a1. 310/9.1 X

[75} Inventor: Frank Massa, Cohasset, Mass. Primary Examiner Mark Q Budd[73] Assignees: Fred M. Dellorfano, Jr.; Donald P.

Massa, both of Cohasset, Mass.

Filed: Aug. 26, 1974 Appl. No.: 500,391

US. Cl. 310/82; 310/83; 3l0/8.7; 310/89; 310/94; 340/8 MM; 340/10 Int.Cl. H01L 41/04 Field of Search 310/82, 8.3, 8.7, 8.9, 3l0/9.l, 9.4;340/10, 8 MM References Cited UNITED STATES PATENTS 9/1947 Bond et a1.310/82 X [57] ABSTRACT formance characteristics of the transducer.

5 Claims, 7 Drawing Figures DIRECTIONAL ULTRASONIC TRANSDUCER WITHREDUCED SECONDARY LOBES frequency region, this invention is particularlyuseful '0 for improving the performance characteristics ofelectroacoustic transducers to be used in the ultrasonic frequencyregion.

Many types of transducer materials such as, for example,magnetostrictive rods, piezoelectric crystals, and polarized ceramicelements, have been widely used in the design of ultrasonic soundgenerators. As is well known to anyone skilled in the art, a designrequirement for achieving reasonably high transducer efficiency atultrasonic frequencies is to operate the transducer element at or nearresonance. If the transducer material employed in the design is apolarized ceramic plate, for example, the operating frequency region ofthe transducer can correspond to the frequency region in the vicinity ofthe thickness resonance of the ceramic plate or, if the ceramic plate isin the form of a circular disc, the planar resonant frequency mode ofvibration of the disc could be used for establishing the frequencyregion of operation.

The use of the transducer materials above mentioned have all proved verysuccessful in connection with the design of underwater transducersbecause the relatively high acoustic impedance of the transducermaterial is reasonably well matched to the relatively high acousticimpedance of the water into which the sound is radiated. However, whensuch a transducer is to be used for generating sound in a gaseousmedium, such as air, the low acoustic impedance of the air is mismatchedconsiderably from the relatively high acoustic impedance of thetransducer material and as a consequence the transducer output andbandwidth are significantly reduced. To overcome these limitations thisinvention provides a novel design of an acoustic transformer which, inaddition to serving as an impedance matching device between thetransducer material and the air, also serves to effectively increase thelimited maximum permissible amplitude of vibration of the transducermaterial whereby the maximum acoustic power radiated into the atmosphereis substantially increased. The novel acoustic transformer designdisclosed in this invention also results in a significant reduction inthe amplitude of the secondary lobes in the directional radiationpattern of the transducer and also in a virtual elimination of thetertiary and higher order lobes thereby additionally improving theperformance characteristics of the transducer.

The primary object of this invention is to improve the radiationefficiency and bandwidth of an ultrasonic transducer which is to operatein a gaseous medium by providing an acoustic transformer between thevibrating surface of the transducer element and the atmosphere intowhich the sound is to be radiated.

Another object of this invention is to improve the acoustic impedancematch between the vibrating sur face of a transducer element and theatmosphere into which the vibrations are to be transmitted.

An additional object of this invention is to provide an acoustictransmission line between the vibrating sur- 2 face of a transducerelement and the atmosphere into which the vibrations are to betransmitted.

A still further object of this invention is to greatly improve theperformance characteristics and power handling capacity of an ultrasonictransducer designed for transmitting acoustic energy into the atmosphereat ultrasonic frequencies.

Another object of this invention is to greatly reduce the secondarylobes in the directional pattern of an ultrasonic transducer whichincludes an acoustic transmission line as a coupling element between thevibrating surface of the transducer element and the atmosphere.

An additional object of this invention is to increase the acoustic poweroutput of an ultrasonic transducer operating in a gaseous medium beyondthe acoustic power output which can be realized from the limitedamplitude of vibration of the surface of the transducer material whenthe vibrating surface is directly exposed to the atmosphere.

In keeping with an aspect of this invention a polarized ceramic disc isoperated at either its planar resonant frequency mode or at itsthickness resonant frequency mode and its efficiency, bandwidth, anddirectional pattern are greatly improved by providing an acoustictransmission line between the vibrating ceramic plate and theatmosphere. The transmission line comprises a material whose specificacoustic impedance is less than the specific acoustic impedance of theceramic and greater than the specific acoustic impedance of air. Thedimensions and configuration of the material are uniquely chosen toachieve the various objects of the invention and the improvedperformance characteristics listed above.

The novel features which are characteristic of the invention are setforth with particularity in the appended claims. However, the inventionitself, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will best be understood byreference to the following description when taken in conjunction withthe accompanying drawings in which:

FIG. 1 is a plan view of a transducer employing the teachings of thisinvention.

FIG. 2 is a vertical section taken along the line 22 of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 2except that the view is taken for the complete transducer.

FIG. 4 is a section taken along the line 44 of FIG. 2 except that thesection is taken for the complete transducer.

FIG. 5 is a plot showing the relative increase in sensitivity achievedby the transducer illustrated in FIG. 2 as a function of I-I/h where His the height of the potting material illustrated in FIG. 2 and A is thewavelength of sound in the potting material at the frequency ofoperation.

FIG. 6 is a plot showing the measured improvement in the transmittingresponse characteristic of an electroacoustic transducer employing theteachings of this invention.

FIG. 7 shows the measured directional pattern of a transducer employingthe teachings of this invention and indicates the virtual elimination ofsecondary lobes.

Referring more particularly to the figures in which one preferred formof the invention is illustrated, l is a housing structure which, forconvenience, is illustrated as being of molded plastic. The plastichousing 1 is represented as a hollow cylinder with three internal ribportions 2. Each of the rib portions is provided with an undercutsection 3 to form a locating nest for accurately positioning the ceramicdisc 4 as illustrated in FIGS. 2 and 3. The ceramic disc 4 may be anyone of the well known polarized ceramic materials such as, for example,lead-zirconate-titanate or barium titanate. The flat faces of the disc 4are provided with metallic electrodes 5 and 6 as is well known in theart. An elec' trical conductor 7 is soldered to electrode surface 5 andelectrical conductor 8 is soldered to electrode surface 6 as illustratedin F IG. 2. A metallic cylindrical collar 9, which includes an extended80 tab member 10, is fitted over the rear projection portion 11 of theplastic housing 1 as illustrated in FIG. 2. The tab portion 10 passesthrough a rectangular slot in the rear portion 11 of the housing andbecomes a terminal post in the vicinity of the ceramic element 4 asillustrated in FIG. 2. The free end of electrical conductor 7 issoldered to the free end of the tab member 10 as illustrated. Acylindrical terminal pin 12 having a shoulder portion 13 and anextension tip portion 14 is pressed through a central hole in the baseportion 11 of the housing 1 as illustrated in FIG. 2. The free end ofelectrical conductor 8 is soldered to the tip portion 14 of the terminalpin 12 as illustrated.

In the embodiment illustrated the external electrical terminalconnections for the ceramic disc appear in the form of a coaxialconnector having a center pin portion 12 and a cylindrical collarportion 9 as illustrated in FIG. 2. To complete the transducer assemblya sound conducting material 15, preferably in the form of a pottingcompound, is used to fill the open end of the housing to provide aheight H of material which is bonded to the electrode surface 5 of theceramic as illustrated in FIG. 2. The height of the acoustic couplingmaterial H is chosen, as will be disclosed, to provide optimumimprovement in the performance char acteristics of the transducer at thedesired frequency.

For the construction illustrated in FIG. 2, the potting material 15, inaddition to serving its primary function as an acoustic transmissionline of length H between the surface 5 of the ceramic disc and theatmosphere, is also used to provide additional mechanical damping to theceramic by permitting it to make direct contact with the peripheral edgesurface of the disc and also with the bottom surface 6. For applicationswhere the transducer is not required to have maximum damping it ispossible to isolate the edge and bottom surfaces of the ceramic discfrom the potting compound by apply ing a layer of low acoustic impedancematerial to either or both of these surfaces prior to potting. Asuitable isolating material is a thin layer of commercially availablefoam rubber or foam plastic tape which may be easily attached to theedge and bottom surfaces of the ceramic disc prior to inserting the discinto position in the housing. The layer of foam tape which may beapplied to the ceramic surfaces as an alternate con struction is notillustrated in the drawings because the description in the text issufficiently clear and it is not necessary to complicate the drawingswhich the illustration of the optional design including the added foamlayer.

In order to achieve increased radiation efficiency and bandwidth for thetransducer, I have found it advantageous to choose a potting material 15whose specific acoustic impedance is greater than the specific acousticimpedance of air and less than the specific acoustic impedance of theceramic material. The specific acoustic impedance is defined as theproduct of density times the velocity of sound in the material. I havealso found that the maximum improvement in sensitivity occurs when theheight of the acoustic coupling material H is made equal to 0.25%, whereis the wavelength of sound in the material 15 at the frequency ofoperation. The increase in sensitivity of a particular transducerincorporating the teachings of this invention as a function of thedimension H is shown in FIG. 5. The data indicates that maximumefficiency occurs at H/)\ 0.25. It also shows that significantimprovement is realized if H/A lies between 0.1 and 0.4. I have alsofound that if the potting material 15 contains a silicon base that thevariation in the specific acoustic impedance of the potting compoundwith temperature can be minimized and the transducer will remain moreuniform in its operational characteristics over a wide range ofenvironmental temperature changes.

FIG. 6 shows the measured transmitting response characteristic of anultrasonic transducer employing a ceramic disc of polarizedlead-zirconate-titanate. Curve 16 shows the measured responsecharacteristic of the transducer when the vibrating surface of theceramic disc is exposed directly to the atmosphere without any couplingmaterial 15 present. Curve 17 shows the improved sensitivity andbandwidth which results when the acoustic coupling material 15 is addedto the ceramic to serve as an acoustic transmission line as illustratedin FIG. 2 and H is made equal to 025A at the operating frequency of thetransducer.

If the transducer employs a ceramic disc 4 which is made of one of thecommon types of polarized lead-zirconate-titanate materials and if thetransducer is operated at or near the planar resonant frequency mode ofvibration of the disc, the beam angle of the directional radiationpattern of the transducer will be approximately 10 to 12 wide at the 3dB points. The measured directional pattern of such a transducerdesignated for operating in the vicinity of 280 kHz is shown in FIG. 7.If barium titanate were chosen as the ceramic material, the planarresonance frequency would be approximately 50 percent higher for thesame size disc and the beam angle would accordingly be somewhat smaller.In any case, by using any of the generally available ceramic materialsfor the disc the beam angle of the transducer will generally be betweenapproximately 8 to 12 at the operating frequency corresponding to theplanar resonant frequency of the disc.

I have also found that by using a height of potting compound H rangingbetween 0.1% and 0.4K that the magnitude of the secondary lobes thatappear in the directional pattern of the transducer are greatly reducedin comparison to the magnitude of the secondary lobes which are normallypresent in the directional radiation pattern of a circular vibratingpiston. FIG. 7 shows the measured directional pattern of a transducerincorporating the teachings of this invention and employing alead-zirconate-titanate ceramic disc operating at the planar resonantfrequency mode. As can be seen in FIG. 7 the secondary lobes are reducedin magnitude by more than 25 dB below the sensitivity on the main axisas compared to a secondary lobe reduction of approximately 17 dB whichis typical for a circular piston. It is also noted in FIG. 7 that anyadditional higher order lobes beyond the secondary lobes are virtuallyeliminated in the directional pattern of the inventive transducer. Theimprovement in secondary lobe reduction results from the internalrefraction of the sound generated by the vibrating ceramic disc as itpasses through the acoustic transmission line 15. The refraction, inturn, is caused by the relatively high velocity of sound in the pottingmaterial as compared to the velocity of sound in air.

The invention has disclosed a novel design of an ultrasonic transducerfor generating sound in air. The teachings of this invention haveresulted in greatly improved performance characteristics of thetransducer. Although specific examples have been described to illustratethe basic teachings of this invention, it will be obvious to anyoneskilled in the art that variations may be made in some of the specificdetails which have been disclosed without departing materially from thenovel teachings of this invention. Therefore, I desire that my inventionshall not be limited except insofar as is made necessary by the priorart and that the appended claims be construed to cover all equivalentstructures.

I claim:

1. in combination in a directional electroacoustic transducer, a housingstructure having an opening, a polarized ceramic disc characterized inthat its periphery is smaller than the periphery of said opening, andfurther characterized in that it operates at a frequency near its planarresonant vibrational mode, said ceramic disc includes a vibratilesurface, means for locating said ceramic disc within said opening insaid housing structure with said vibratile surface placed opposite saidopening, means for reducing the amplitude of the secondary lobes in thedirectional pattern of said transducer, said means including a soundconducting material located within said opening and making intimatecontact with said vibratile surface.

2. The invention in claim 1 characterized in that the thickness of saidsound conducting material lies in the range 1/ 10 to 4/10 wavelength ofthe sound generated in said sound conducting material at the planarresonant vibrational mode of said ceramic disc.

3. The invention in claim 2 further characterized in that said soundconducting material also makes intimate contact with the peripheral edgesurface of said ceramic disc.

4. The invention of claim 2 further characterized in that said soundconducting material is an elastomer.

5. The invention in claim 4 further characterized in that said elastomercontains silicone.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.5,928,777 Dated December 25, 1975 Inventor (s) Frank Massa It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Item (75) should read;

Fred M. Dellorfano, Jr. and Donald P. Massa, Trustees of the StoneleighTrust u/d/t December i, 1975, Cohasset, Massachusetts ,lzigncd andScaled this Twenty-first D a y f September 1976 SE A L l A ttes t:

RUTH C. MASON Commissioner ofParents and Trademarks UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No- 5,928,777 Dated December 25,1975 Inventor(s) Frank Massa It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Item (75) should read;

Signed and Sealed this Twenty-first D f September 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflarenrsand Trademarks

1. In combination in a directional electroacoustic transducer, a housingstructure having an opening, a polarized ceramic disc characterized inthat its periphery is smaller than the periphery of said opening, andfurther characterized in that it operates at a frequency near its planarresonant vibrational mode, said ceramic disc includes a vibratilesurface, means for locating said ceramic disc within said opening insaid housing structure with said vibratile surface placed opposite saidopening, means for reducing the amplitude of the secondary lobes in thedirectional pattern of said transducer, said means including a soundconducting material located within said opening and making intimatecontact with said vibratile surface.
 2. The invention in claim 1characterized in that the thickness of said sound conducting materiallies in the range 1/10 to 4/10 wavelength of the sound generated in saidsound conducting material at the planar resonant vibrational mode ofsaid ceramic disc.
 3. The invention in claim 2 further characterized inthat said sound conducting material also makes intimate contact with theperipheral edge surface of said ceramic disc.
 4. The invention of claim2 further characterized in that said sound conducting material is anelastomer.
 5. The invention in claim 4 further characterized in thatsaid elastomer contains silicone.